It is known, as disclosed for example in U.S. Pat. Nos. 4,286,436 and 4,335,581, to employ a tube and shell heat exchanger in processes for freeze concentrating liquid mixtures. In such use, the exchanger typically consists of a plurality of vertical freeze tubes through which the mixture to be concentrated passes, and an external shell which, together with appropriate tube sheets at the upper and lower ends of the freeze tubes, defines a chamber within which an appropriate cooling fluid, typically a liquefied normally gaseous cryogen or refrigerant, such as ammonia or a Freon refrigerant, is contained. The cryogen is maintained at its boiling point at the pressure conditions which exist within the shell. Heat extracted from the liquid mixture passing through the freeze tubes converts a portion of the liquid cryogen to vapor, which is withdrawn from the shell. Additional liquid cryogen is supplied to the shell as needed to replace that withdrawn as vapor.
In the operation of such a freeze concentrator, it is important that the temperature of the liquid cryogen be maintained at a constant value, for several reasons. If the temperature falls below a design value, either within the entire pool or at localized areas along the length of the freeze tubes, the solvent or liquid carrier in the mixture passing through the freeze tubes may freeze at a rate higher than intended and deposit on the tube walls, thus leading to blockage of the freeze tubes. In addition, the existence of localized zones of different refrigerant temperature will result in uneven heat transfer, and thus in an overall reduction in the cooling capacity of the freeze exchanger. Accordingly, there exists a need for a freeze concentrator of the tube and shell type in which the temperature of the cryogen can be maintained at a desired constant value in a simple and economical manner requiring a minimum of process controls.